Numerous devices for demounting a tire from a rim have already been proposed, such devices being installed on a tire mounting-demounting machine. More particularly, they are supported at the projecting end of a telescopic support arm slidably mounted in any suitable manner on guides, which, in the case of a vertical tire mounting-demounting machine, are borne by or are integral with an upright or column which upwardly extends on the rear of the base of the tire mounting-demounting machine.
The support arm is equipped with means for controlling both the lifting-lowering thereof and in order to carry out controlled movements of extension-withdrawal with respect to the upright or column. Below the telescopic support arm, the tire mounting-demounting machine supports, on its own base, rotatable rim-carrier means, such as a table equipped with retention jaws or a group of expandable-retractable arms or the like, for engaging a rim or a tired wheel and driving it in controlled rotation.
The patent IT-1 381 936 in the name of the Applicant of the present patent application teaches a work tool articulated to the projecting end of a telescopic support arm, as specified above, by means of an extension element which is connected to the work tool with the interposition of articulation means adapted to allow the articulation between the extension and the tool in accordance with a thrust force applied to the tool. The articulation means comprise an articulation pin between the extension element and the tool, an elbow-shaped lever pivoted in an intermediate point thereof on the articulation pin and having one end thereof pivoted to the tool and the other end thereof pivoted to one end of a gas spring, whose other end is articulated to an intermediate point of the support arm. The function of the gas spring is that of yieldably opposing the articulation between the tool and the extension element, so as to prevent the tool from being tilted and thus not working the moment it is brought into contact with the flank or bead of a tire to be demounted.
A tool structured like that taught in the patent IT-1 381 936 operates in a very satisfactory manner with most of the tires on the market, but it does not work as well with tires equipped with bead-saver edge. Indeed, due to the relative rigidity of the articulation between the tool and the extension ensured by the presence of the gas spring, the tool—when it is brought against a bead of a tire equipped with bead-saver edge, which is an edge in relief that in use comes to be situated in abutment against the edge of the rim—upon receiving the lowering thrust of the support arm, slides backwards on the bead-saver edge, i.e. instead of inserting its own tip between the bead and the rim, it is moved away from the rim. After this, the tip is prevented from reaching the zone between bead and rim, due to the presence of the bead-saver edge which it is no longer able to pass over. This blocks the continuation of the automatic demounting operation, and requires the manual intervention of the operator on the tool, with consequent downtime in the tire maintenance operation.
Similar problems can also be encountered with other tire types: during the demounting steps, the tool can be stuck on the side of the tire and thus it will not be able to correctly complete the demounting operations. With soft tires in particular, the demounting tool might be driven into the side of the tire during demounting, so that the tool is stuck on the side of the tire, in which case it will be necessary to return the tool to a position distant from the tire and restart the demounting cycle.
EP-1 593 533 and WO/2009-138 322 teach respective machines for demounting a tire from a wheel rim.